An ideal amplifier exhibits a linear transfer characteristic. For real amplifiers, biasing is a technique used to achieve linear operation. This can be achieved by biasing the circuit by applying a direct current (DC) voltage or current to select an operating point, or the quiescent point. The signal to be amplified is then superimposed on the DC bias. The bias point should be set high enough so that the amplifier does not enter a cut-off state while amplifying the signal, and yet low enough so that the amplifier does not saturate.
Crossover distortion occurs in circuits that use devices operating in a “push-pull” manner. The devices are used in pairs and each device operates for only a half cycle of the input signal (e.g., class AB operation). However, if the circuit is not designed to achieve a smooth crossover or transition from one device to another, then there can be a non-linear region of the transfer characteristics (e.g., when the output has a zero amplitude for a non-zero input). The biasing of each device can determine the amount of time that the device operates over the input signal's time period.